Question

I have this records in my table :

id   |   title     |  sub
-----+-------------+------
1    | Parent 1    | 0
2    | Parent 2    | 0
3    | Sub 1-1     | 1
4    | Parent 3    | 0
5    | Sub 1-2     | 1
6    | Sub 2-1     | 2
7    | Parent 4    | 0

Now I want select this records from my table with one query like this :

id   |   title     |  sub
-----+-------------+------
1    | Parent 1    | 0
3    | Sub 1-1     | 1
5    | Sub 1-2     | 1
2    | Parent 2    | 0
6    | Sub 2-1     | 2
4    | Parent 3    | 0
7    | Parent 4    | 0

I want to sort my records by Parent and then childs.

How can I do this in mySQL ?

UPDATE:

I use this query :

SELECT a.*, 
       CASE WHEN SUB = 0 THEN ID ELSE SUB END expression
  FROM category a
 ORDER BY CASE WHEN SUB = 0 THEN ID ELSE SUB END, ID

And my data is :

id  | title     | sub 
----+---------------+-------
1   | Parent 1  | 0
2   | Parent 2  | 0
3   | Sub 1-1   | 7
4   | Parent 3  | 0
5   | Sub 1-2   | 4
6   | Sub 2-1   | 2
7   | Parent 4  | 0

The result is :

id  | title     | sub   | expression
----+-----------+-------+-----------
1   | Parent 1  | 0     | 1
2   | Parent 2  | 0     | 2
6   | Sub 2-1   | 2     | 2
4   | Parent 3  | 0     | 4
5   | Sub 1-2   | 4     | 4
3   | Sub 1-1   | 7     | 7
7   | Parent 4  | 0     | 7

Answer 1

If there wouldn’t be more than 1 level, you could make an expression with CASE on your order by clause:

SELECT a.*, 
       CASE WHEN SUB = 0 THEN ID ELSE SUB END expression
  FROM test a
 ORDER BY CASE WHEN SUB = 0 THEN ID ELSE SUB END, SUB, TITLE

With that expression, it will always order by the parentId, which is the SUB column when populated or the ID column when it is not a child.

Would result:

ID  TITLE        SUB   EXPRESSION
1   Parent 1       0          1
3   Sub 1-1        1          1
5   Sub 1-2        1          1
2   Parent 2       0          2
6   Sub 2-1        2          2
4   Parent 3       0          4
7   Parent 4       0          7

It would still fail if any of the Child could be a parent of another record (Sub 1-2-1 for example).